THE ELBOW JOINT AND THE MUSCLES OF THE ARM
The skeletal basis of the elbow joint is provided by the distal end of the humerus and proximal parts of the radius and ulna (Figure 23-6, A). Both epicondyles of the humerus may be palpated without much difficulty, but the medial one is especially prominent and projects to the inner aspect of the olecranon.
The condyle may be identified more distally; it presents a deep fossa into which fits the anconeal process of the olecranon (Figure 23-9/4,6). A shallow radial fossa occupies the corresponding site on the cranial aspect.The powerful olecranon rises high above the joint to project on the lower part of the fifth rib (or following space) and is therefore a less direct guide to the position of the articulation. The shaft of the ulna is much reduced. It tapers distally to fusion and ultimate submergence within the shaft of the radius, but it leaves open an interosseous space in the proximal forearm. The proximal extremity of the radius is expanded. It carries an articular surface that engages with the cylindrical humeral condyle and, just distal to this, medial and lateral eminences that furnish attachment to the collateral ligaments. The radial tuberosity is present to the front (Figure 23-9/8). Both collateral ligaments may be palpated, although the medial one is covered by the relatively thick pectoralis transversus. A cranial division of this ligament represents a vestige of the pronator teres.
The shape of the articular surfaces and the presence of stout collateral ligaments restrict movement of the elbow joint to flexion and extension in a sagittal plane. The equine elbow is a good example of the “snap” joint, which abruptly moves from a stable to a more mobile position. This character depends on two features of its construction. The first is the unequal curvature of the humeral surface; the radius of curvature of the central part is longer than those of the parts in front and behind, which are in contact with the radius in the more flexed and more extended positions of the joint. The second is that the collateral ligaments insert eccentrically on the humerus and are taut only in the intermediate position (Figure 23-10).
The joint is most conveniently punctured by passing a needle between the lateral epicondyle and the olecranon into a caudal pouching of the joint capsule within the olecranon fossa.
The muscles of the arm that operate the elbow joint are arranged in flexor and extensor groups.